1. Field of the Invention
This invention relates generally to signal processing systems, and relates more particularly to a system and method for effectively implementing fixed masking thresholds in an audio encoder device.
2. Description of the Background Art
Providing an effective method of encoding audio data is often a significant consideration for designers, manufacturers, and users of contemporary electronic systems. Developments in modern digital audio technology have necessitated corresponding improvements in sophisticated, high-performance audio encoding methodologies. For example, the operation of recordable audio compact-disc devices typically requires an encoder-decoder (codec) system to receive and encode source audio data into a format (such as MPEG) that may then be recorded onto appropriate media using the compact-disc device.
Referring now to FIG. 1, a block diagram for one embodiment of an audio encoder-decoder (codec) 110 is shown. In the FIG. 1 embodiment, codec 110 comprises a decoder 114 and an encoder 112 that includes a psycho-acoustic modeler (PAM) 126. During an encoding operation, encoder 112 receives source audio data from any compatible audio source via path 116, responsively filters the source audio into frequency sub-bands, and then generates encoded audio data that may be provided to an audio device (such as a recordable compact-disc device or a computer system) via path 138. The operation of psycho-acoustic modeler (PAM) 126 is further discussed below in conjunction with FIG. 2.
Referring now to FIG. 2, a graph 210 for one embodiment of exemplary masking thresholds for the FIG. 1 encoder-decoder system 110 is shown. Graph 210 displays audio data signal energy on vertical axis 212, and also displays a series of frequency sub-bands on horizontal axis 214. In operation,. psycho-acoustic modeler (PAM) 126 receives source audio data, and then utilizes characteristics of human hearing to generate the masking thresholds 228. Experiments have determined that human hearing cannot detect some sounds of lower energy when those lower energy sounds are close in frequency to sounds of higher energy.
For example, sub-band 3 (220) includes a 60 db sound 232, a 30 db sound 234, and a masking threshold 230 of 36 db. The 30 db sound 234 falls below masking threshold 230, and is therefore not detectable by the human ear due to the masking effect of the 60 db sound 232. In practice, encoder 112 may thus discard any sounds that fall below masking thresholds 228 to advantageously reduce the amount of audio data and expedite the encoding process.
Psycho-acoustic modeler 126 thus provides useful information for reducing the amount of audio data that must be encoded by encoder 112. However, implementing psycho-acoustic modeler 126 within encoder 112 substantially increases the complexity of encoder 112, and also approximately doubles the processing power required to control encoder 112. The cost and difficulty of successfully implementing psycho-acoustic modeler 126 are therefore significant negative aspects of the FIG. 1 encoder-decoder system 110. An encoder device that exhibits reduced complexity, while still achieving acceptable quality in the encoded audio data would thus provide distinct advantages to system manufacturers and users. Therefore, for all the foregoing reasons, an improved system and method are needed to effectively implement fixed masking thresholds in an audio encoder device.
In accordance with the present invention, a system and method are disclosed for effectively implementing fixed masking thresholds in an audio encoder device. In one embodiment of the present invention, system designers of the encoder initially create a masking threshold lookup table. The masking threshold lookup table may include masking threshold values that are based upon empirically-derived absolute human hearing thresholds. In alternate embodiments, the lookup table may similarly include masking thresholds that are selectively tuned to deviate from the absolute human hearing thresholds.
Next, a filter bank in the encoder receives and filters source audio data into frequency sub-bands to provide filtered audio data to a bit allocator. The bit allocator then responsively analyzes the filtered audio data using the masking thresholds contained in the lookup table. Specifically, the bit allocator identifies masked audio data to be any filtered audio data that falls below the fixed masking thresholds from the lookup table. Similarly, the bit allocator identifies any filtered audio data that lies above the fixed masking thresholds in the lookup table as non-masked audio data.
The bit allocator may then discard the filtered audio data that was identified as masked audio data to advantageously decrease the total amount of filtered audio data to be processed by the encoder. Next, the bit allocator allocates all available allocation bits to the filtered audio data that was previously identified as non-masked audio data to generate allocated audio data to a quantizer.
In response, the quantizer quantizes the allocated audio data to generate quantized audio data to a bitstream packer. Finally, the bitstream packer packs the quantized audio data to produce encoded audio data for storage onto an appropriate and compatible storage medium, in accordance with the present invention. The present invention thus efficiently and effectively provides a system and method for effectively implementing fixed masking thresholds in an audio encoder device.